Improve warning message when the GEM_HOME/GEM_PATH are misconfigured

Author: eregon

src/main/ruby/truffleruby/core/truffle/gem_util.rb

@@ -92,8 +92,10 @@ def self.upgraded_default_gem?(feature)
   def self.verify_gem_paths
     bad_dirs = bad_gem_dirs(gem_paths)
     unless bad_dirs.empty?
-      warn "[ruby] WARNING gem paths: #{bad_dirs.join ', '} are not marked as installed by TruffleRuby " +
-               '(they could belong to another Ruby implementation and break unexpectedly)'
+      warn "[ruby] WARNING gem paths: #{bad_dirs.join ', '} are not marked as installed by TruffleRuby. " +
+               'They might belong to another Ruby implementation and break unexpectedly. ' +
+               'Configure your Ruby manager to use TruffleRuby, or `unset GEM_HOME GEM_PATH`. ' +
+               'See https://github.com/oracle/truffleruby/blob/master/doc/user/ruby-managers.md'
     end
     bad_dirs
   end

tool/jt.rb

@@ -2312,6 +2312,7 @@ def check_documentation_urls
       https://github.com/oracle/truffleruby/blob/master/doc/user/installing-libssl.md
       https://github.com/oracle/truffleruby/blob/master/doc/user/installing-llvm.md
       https://github.com/oracle/truffleruby/blob/master/doc/user/installing-zlib.md
+      https://github.com/oracle/truffleruby/blob/master/doc/user/ruby-managers.md
     ]
 
     known_hardcoded_urls.each do |url|